Doohan had a chip on his shoulder that he made well known, because he saw himself as this highly multi-talented actor who was unable to realize his potential on the show, because his role was quite small compared to Captain Kirk and Spock. But just look at what happened to him afterward. He really didn't do anything noteworthy, not one production outside of Star Trek that you could easily recall off the top of your head.

Jason of Star Command
Loaded Weapon
The Ben Stiller Show

That's all I got.

He also did a stint on the soap opera The Bold and Beautiful.

Thing is, even if the junior cast members weren't on TV/movies all the time, that doesn't mean they weren't working. Some did live theatre, others wrote, Nichelle Nichols did some work as a recruiter for NASA, to find qualified women/minority people to train as astronauts, Takei took a shot at politics, and so on.

Walter Koenig wrote plays and performed on stage, he wrote comic book scripts, he acted... I don't think anybody here would say he wasn't pretty damn good on Babylon 5.

And why shouldn't these people fight for their characters and resent every instance of Shatner thoughtlessly (not necessarily deliberately) taking lines that could be said by someone else? They had the right to be concerned for their jobs, too!